Nuclear electronics is the body of methods of nuclear physics in which electronic instruments are used to record, convert and process the information coming from particle detectors. The short duration of the processes and, as a rule, their high frequency, together with the presence of extraneous processes (background), demand of nuclear-electronic instruments a high time resolution (~10-9 s). The need to measure a large number of parameters at the same time (the amplitude of the signal, its time of arrival, the coordinate of the point at which it is detected and so on) meant that it was in nuclear electronics that analogue-to-digital conversion circuits were first developed, and that digital methods of accumulating information and multichannel and multidimensional analysis by computer were first applied.
In the recording of particles, and also of the photons of X-ray and γ-radiation, the task of nuclear electronics reduces to counting the pulses from the detector; in identifying the type of particle or in studying its spectrum, the shape of the pulse, its amplitude or the relative delay between pulses is analysed. When the spatial distribution of the particles is studied, the numbers of the detectors that have “triggered” are recorded, or the coordinate of the point of detection is determined directly. Nuclear-electronic devices use anticoincidence and coincidence methods, amplitude discriminators, linear gates and adders, multichannel time and amplitude analysers, and also devices for reading information out of position-sensitive detectors (spark chambers and proportional chambers), and so on.
A device for recording particles contains: a detector; a signal amplifier; a converter, which turns the detector signal into a standard pulse or converts the amplitude or the time of arrival of the signal into a digital code; and a recording instrument (pulse counters, memory units, a computer, more rarely self-recording instruments or photographic equipment). Fig. 1 shows a simplified system for studying the spectrum of particles. A charged particle crosses the detectors D1 — D3 and stops in the detector D4. The signals from D1—D3 pass through the pulse shapers S1, S2, S3 to the coincidence circuit CC, which selects those events in which the signals arrive at its inputs at the same time. The simultaneous arrival of the pulses is secured by matched delay lines DL. The coincidence circuit produces a signal that “enables” the conversion of the pulse under study from the detector D4. The result of the conversion is entered from the analogue-to-digital converter ADC, as a digital code, into the random-access memory RAM or into a computer. The measured amplitude spectrum is displayed on the screen of the cathode-ray tube CRT. The part of the system bounded by the dotted line is a multichannel pulse-amplitude analyser. The counting rate at the output of the coincidence circuit, registered by the counter Ct, gives the number of recorded events. The time selection of the signals is carried out by coincidence circuits, which are triggered by pulses of a definite duration and amplitude.
Amplitude selection uses discriminators, which generate an output pulse if the amplitude of the input signal is either greater than a definite value (an integral discriminator) or lies within definite limits (a differential discriminator). Discriminators are built as a Schmitt trigger (a trigger circuit) or with comparison circuits (comparators) made in the form of integrated circuits. The latter are high-sensitivity limiting amplifiers.
In modern nuclear electronics, units that perform a single logic function (“AND”, “OR” and others) are giving way to universal multifunction devices whose logic functions can be set from outside. Such devices are built on read-only memories, programmable logic arrays or gate arrays. Computing has made it possible to create automated equipment with parameters controlled by program: the computer controls the triggering thresholds of the circuits, the time resolution, the delay of the signals, the logic by which events are selected, the operating mode of the measuring system and so on. Microprocessors, and special processors for pattern recognition and for the accumulation and preliminary processing of measurement results, are being introduced into experiment. Experimental data are accumulated in the computer and then written to magnetic tape. The results of the preliminary processing are displayed on the screen of a cathode-ray tube, which allows the operator to intervene in the course of the measurements. The computer controls various actuating devices: motors that move the detectors or the targets, relays, signal switches and so on (Fig. 2).